Obviously you need to accept your own answer hankerot for future people to learn of your solution. I submitted your answer as a solution to the problem to moderation. My suggestion was denied. I'll try resubmitting it, we will see where it goes.

2 Answers

My theory: The "inverter" is a switching powersupply with low voltage DC input, chopping, transforming up (coils) to produce some higher voltage DC for the backlight lamp. If the lamp has a short, the PSupply runs into current limitation with the output voltage collapsing. Hence, the backlight lamp must be replaced --- or a new display must be purchased.

Back to reality: A new display is bought. I ordered the same Samsung type (LTN133 AT002-001), but got a Toshiba (LTN133 other-bla). Dismantling once more is required, because the power-plug has to go into the inverter. Now, TEST the Macbook with everything dangling, i.e. held by sticky tape. - Boot-up - VOILA, the Macbook is a fully functioning laptop with high-gloss screen. The re-assembling is not a big adventure anymore (you might call it "experience").

My daughter has her beloved Mac back; I am grateful to everybody in the forum, who gave useful hints.

PS. I keep the inverter. It will never break as long as i have a spare.

This sounds like an inverter problem or inverter cable problem. Hook it up to an external monitor to check this out. To get you to the correct part and installation guide please give the last three figures of your serial number, (found in your battery compartment). Thanks

All experts point to a broken "inverter". I bought one from IFIXIT (good service!).

I replaced the inverter using the "Fast Method" - worked fine. It requires care and "feeling" to get the cover back on with the keyboard in place.

My advice: First, move cables on both sides with tweezers "behind" the hinge-rods out of the way of the U-cover sliding over. Second, the inverter PCB has a coil-assembly mounted. It is in the way of the lower lip of the U-shape. To ease mounting, i clipped off the lower "guides" on either side. Then, with a bit of force (NOT brute) and bending the U wider, the cover goes on. The guides are not needed, because the cover is rigid and held by 3 screws to the display lid.

So far, so perfect: Run up the Macbook. The bitten apple shows up briefly (1 sec) at boot-time; and the display goes dark. Running "brightness" to "0" and back up illuminates display (again 1 sec) followed by darkness. The new inverter shows exactly the same behaviour as the old ("faulty") one - disappointment.